1. Field of the Invention
The present invention generally relates to a system and a method of light collection, suitable for application to a light energy conversion mechanism in a solar cell module or for converting solar energy.
2. Description of Related Art
A light collection system collects a great number of incident lights and then emits the lights in a denser form through a smaller optical region. A light collection system may be applied to a solar cell and the heat collection of solar energy.
A solar cell needs to absorb an incident light effectively. However, a portion of the incident light is reflected due to the nature physical phenomena. FIG. 1 illustrates optical refraction and reflection. Referring to FIG. 1, a transparent layer 100 having a refractive index n receives an incident light 102a through the air. Based on the optical refraction phenomenon, a refracted light 102b enters the transparent layer 100 and a reflected light 102c is reflected according to the incident angle θ of the incident light 102a. FIG. 2 illustrates the relationship between an incident angle and a reflectivity. Glass and air interface are taken as an example. Referring to FIG. 2, regarding the reflection of a light outside of a medium, when the incident angle θ is greater than a specific angle (for example, greater than 60°, the reflection is considerably increased, and accordingly the reflected light 102c gets more and more intense. In other words, the less refracted light 102b enters the transparent layer 100, the lower the optical efficiency is.
FIG. 3 illustrates the relationship between an incident angle of a light and the absorption response of a solar cell. Referring to FIG. 3, the absorption corresponding to an incident angle of 0° will be described as an example. When the incident angle is greater than 50°, the absorption of the solar cell starts to decrease drastically. Accordingly, in an unsatisfactory light collection design, if there are too many lights entering a solar cell at greater angles, the photovoltaic conversion efficiency of the solar cell will be reduced due to the unsatisfactory absorption efficiency thereof. In other words, the optical design of a highly-efficient light collection system in a solar cell has to allow most incident lights to enter the solar cell at angles smaller than 50°. However, the disadvantage of such a design falls on the some kinds of low-f-number solar cell light collectors.
Various designs of low-f-number solar concentrator have been provided. FIG. 4 illustrates the structure of a low-f-number solar cell light collector. Referring to FIG. 4, the light collector 400 of a solar cell receives an incident light 401. After the incident light 401 enters a first light collector 402, the low-f-number light collector 400 reflects the incident light 401 based on a total internal reflection (TIR) sawtooth structure 403, and the output light 404 is refracted at a central secondary optical element 405 and is uniformly distributed on the solar cell 406. As a result, a compact design and a high light concentration ratio are both achieved.
However, in foregoing low-f-number light collector 400, most lights enter the solar cell 406 at large angles, and these lights are mostly entered in a large optical entrance area, for example, the outer tooth structure of TIR optics. Besides, sharp teeth 407 have to be designed in the TIR sawtooth structure 403 of the light collector 400 in order to achieve a high optical efficiency. However, in an actual injection molding process, these saw teeth may produce round corner. Accordingly, the optical efficiency of the light collector 400 may be reduced.
FIG. 5 illustrates another refraction-reflection-TIR (RXI) short-focal-length solar light collector. After an incident light 501 enters the solid light collector 500, it is first reflected by a bottom mirror 502, then internal totally reflected by the surface 505 of the light collector 500, and eventually irradiated onto the solar cell 504. An incident light closer to the center is first reflected by the bottom mirror 502 and then reflected by a top mirror 503 before it enters the solar cell 504.
Foregoing RXI compact solar light collector 500 offers both high light collection ratio and very short focal length. However, in this design, the incident light mostly enters the solar cell at a very large angle after it is totally reflected, and accordingly the photovoltaic conversion efficiency of the solar cell is reduced.
FIG. 6 illustrates a conventional Cassegrain reflective solar optical focusing device. In a light collection system 600, after an incident light 601 enters a first reflection surface 602, it is reflected to a second reflection surface 603, then focused on a CPC color mixing element 604, and eventually refracted to a solar cell 605.
Foregoing Cassegrain light collection system 600 offers both a compact design and a high light concentration ratio. In addition, a smaller incident angle and a more uniform irradiance can be obtained through the secondary refractive element 604. However, the assembly tolerance between the primary element 602 and a secondary element 603 in such a light concentrating device is extremely low. Namely, the irradiance of the solar cell required needs very high assembly precision between mirrors and to the color mixing element.
FIG. 7 illustrates another conventional Fresnel light collection system. A most commonly used refractive light collection system 700 has a primary light collection system, wherein the primary light collection system may be a Fresnel lens 701 and which concentrates an incident light at a focus 702 and then collects the light onto a solar cell 704 through direct irradiation or reflection of a secondary optical element 703.
FIG. 8 illustrates the distribution of irradiance on a cross section of a solar cell. As shown in FIG. 8, the received light irradiance is very uneven, and there is a strong light concentration effect in the center area. Such an uneven distribution may cause uneven temperature and irradiance on the solar cell, and accordingly the heat dissipation and the photovoltaic conversion efficiency of the solar cell may be reduced.
There are still many different designs provided regarding different problems in conventional light collection systems. However, the conventional light collection systems have their disadvantages. For example, the low photovoltaic conversion efficiency of a solar cell caused by uneven irradiance, the low absorption and photovoltaic conversion efficiency of a solar cell caused by large incident light angles, the affection of process precision and assembly tolerance to irradiance, and the acceptance angles of the optics are all problems to be resolved in the conventional technique. Accordingly, a better design of light collection system is still to be provided.